The present invention relates to a liquid ejection apparatus for ejecting liquid droplets from nozzle orifices, and particularly relates to a liquid ejection apparatus for ejecting liquid droplets from a plurality of nozzle orifices during each of reciprocating motions thereof.
In an ink jet recording apparatus (kind of the liquid ejection apparatus) such as an ink jet printer or an ink jet plotter, a recording head (head member) is moved in a primary scanning direction while recording paper (kind of liquid-ejected medium) is moved in a secondary scanning direction. In connection with such motions, ink droplets are ejected from nozzle orifices of the recording head so as to record an image (including characters and so on) on the recording paper. The ejection of ink droplets is performed, for example, by expansion and contraction of pressure generating chambers communicating with the nozzle orifices.
The expansion and contraction of the pressure generating chambers are performed, for example, by use of deformation of piezoelectric vibrators. In such a recording head, each piezoelectric vibrator is deformed in response to a driving pulse supplied thereto so that the volume of its corresponding pressure chamber is varied. In response to the volume change, there occurs a change of pressure in ink in the pressure chamber. Thus, an ink droplet is ejected from the nozzle orifice communicating with the pressure chamber.
In such a recording apparatus, a drive signal having a plurality of pulse waveforms connected in series is generated. On the other hand, print data SI including gradation information is transmitted to the recording head. Then, in accordance with the transmitted print data SI, only required pulse waveforms are selected from the drive signal and supplied to the piezoelectric vibrator. Thus, the quantity of an ink droplet to be ejected from the nozzle orifice is changed in accordance with the gradation information.
More specifically, for example, in a printer in which four gradations of non-recording print data (gradation information 00), small-dot print data (gradation information 01), middle-dot print data (gradation information 10) and large-dot print data (gradation information 11) are set, ink droplets different in ink volume are ejected in accordance with the gradation levels respectively.
In order to attain four-gradation recording as described above, for example, a drive signal PA as shown in FIG. 21 can be used. This drive signal PA is a pulse train waveform signal in which a first pulse signal PAPS1 disposed in a period PAT1 and a second pulse signal PAPS2 disposed in a period PAT2 are connected in series and which is generated repetitively with a recording period PATA.
In the drive signal PA, the first pulse signal PAPS1 is a small-dot driving pulse for ejecting a small ink droplet from a nozzle orifice, and the second pulse signal PAPS2 is a middle-dot driving pulse for ejecting a middle ink droplet from a nozzle orifice.
In this case, as shown in FIG. 22, recording corresponding to the large dot can be performed by supplying a combination of the first pulse signal PAPS1 and the second pulse signal PAPS2.
In order to perform recording on recording paper at a higher speed, it is preferable that ink droplets are ejected from the nozzle orifices of the recording head to thereby record an image (including characters and so on) on the recording paper in each of forward travel and backward travel of reciprocating motion of the recording head in the primary scanning direction. That is, it is preferable that after recording one line during forward motion, the recording head moves by line width (including interline width) in the secondary scanning direction relatively to the recording paper, and records the next line during backward motion (in an opposite direction). The ink jet recording apparatus capable of recording in each of forward and backward motions is called a bi-directional (Bi-D) type.
In order to improve the recording accuracy in the bi-directional type ink jet recording apparatus, it is known that the waveform of a forward drive signal is preferably made different from the waveform of a backward drive signal. Generation of such waveforms of drive signals is described in detail in Japanese Patent Publication No. 2000-1001A.
An example will be described with reference to FIGS. 23A and 23B. A forward drive signal PA is a periodic signal of a first pulse train P1 having a first pulse waveform w1 and a second pulse waveform w2 in that order.
Here, the first pulse waveform w1 and the second pulse waveform w2 correspond to the first pulse signal PAPS1 and the second pulse signal PAPS2 in FIG. 21 respectively. That is, the first pulse waveform w1 (first pulse signal PAPS1) is a pulse waveform for ejecting a small-dot liquid droplet, and the second pulse waveform w2 (second pulse signal PAPS1) is a pulse waveform for ejecting a middle-dot liquid droplet.
Then, two-bit pulse selection data is generated in accordance with gradation data per recording pixel during forward motion. In this case, pulse selection data (10) for selecting only the first pulse waveform w1 is generated in accordance with gradation data corresponding to a small dot; pulse selection data (01) for selecting only the second pulse waveform w2 is generated in accordance with gradation data corresponding to a middle dot; and pulse selection data (11) for selecting both the first pulse waveform w1 and the second pulse waveform w2 is generated in accordance with gradation data corresponding to a large dot.
On the other hand, a backward drive signal PB is a periodic signal of a second pulse train P2 having a second pulse waveform w2 and a first pulse waveform w1 in that order. Here, the second pulse waveform w2 and the first pulse waveform w1 are similar to those of the forward drive signal PA.
Then, two-bit pulse selection data is generated in accordance with gradation data per recording pixel during backward motion. In this case, pulse selection data (01) for selecting only the first pulse waveform w1 is generated in accordance with gradation data corresponding to a small dot; pulse selection data (10) for selecting only the second pulse waveform w2 is generated in accordance with gradation data corresponding to a middle dot; and pulse selection data (11) for selecting both the first pulse waveform w1 and the second pulse waveform w2 is generated in accordance with gradation data corresponding to a large dot.
In such a manner, the order of the pulse waveforms belonging to the forward drive signal is made reverse to the order of the pulse waveforms belonging to the backward drive signal. Thus, as shown in FIG. 24, the positions (in the primary scanning direction) where ejected ink droplets are landed can be aligned in the secondary scanning direction.
In addition, each ink droplet ejected during the forward motion has an initial velocity in which a forward velocity component of the recording head is added to the ink droplet's own initial velocity from the recording head toward the recording paper. Therefore, the point where the ejected ink droplet is landed actually on the recording paper is shifted in the forward direction. On the contrary, each ink droplet ejected during the backward motion has an initial velocity in which a backward velocity component of the recording head is added to the ink droplet's own initial velocity from the recording head toward the recording paper. Therefore, the point where the ejected ink droplet is landed actually on the recording paper is shifted in the backward direction. Thus, in order to secure continuity between a subject (for example, an image) to be recorded during the forward motion and a subject to be recorded during the backward motion, adjustment is made such that the timing with which the backward drive signal is supplied is evenly shifted from the timing with which the forward drive signal is supplied. This shift quantity is called a Bi-D adjustment value.
Determination of the Bi-D adjustment value (timing adjustment value) is made by printing a vertical ruled line during forward motion and backward motion following the forward motion to thereby verify continuity, or printing a patch pattern during forward motion and backward motion following the forward motion to thereby examine the presence/absence of a sense of surface roughness.
On the other hand, in a recording head for color printing, a plurality of arrays of nozzle orifices for ejecting a plurality of color inks respectively are provided in parallel. Desired color recording can be obtained by ejecting the respective colors of ink suitably on top of one another. The plurality of color inks are, for example, black ink, cyan ink, magenta ink, and yellow ink.
Generally, in bi-directional type color ink jet recording apparatus, a Bi-D adjustment value for the black ink and a Bi-D adjustment value for the other color inks are adjusted independently.
However, in order to attain higher-quality color printing, the bi-directional type color ink jet recording apparatus as described above has the following problems.
For example, assume that in a recording head for color printing, an array of nozzle orifices for ejecting cyan ink (C), an array of nozzle orifices for ejecting magenta ink (M) and an array of nozzle orifices for ejecting yellow ink (Y) are provided in parallel in that order, and recording is carried out with the cyan ink (C), the magenta ink (M) and the yellow ink (Y) in that order during the forward motion of the recording head. In this case, during the backward motion of the recording head, recording is made with the yellow ink (Y), the magenta ink (M) and the cyan ink (C) in that order.
Here, consideration is given to gray color formed in a three-color composite of the cyan ink (C), the magenta ink (M) and the yellow ink (Y). In the forward motion of the recording head, the cyan ink (C), the gray color is formed by superimposition of the magenta ink (M) and the yellow ink (Y) on one another in that order. On the contrary, in the backward motion of the recording head, the gray color is formed by superimposition of the yellow ink (Y), the magenta ink (M) and the cyan ink (C) on one another in that order.
It is known that one and the same combination of inks may produce different tones due to a difference in the order in which the ink droplets are landed, as described the above. A variation (shift) of a tone caused by the order in which inks are landed is the most conspicuous in gray color, particularly a halftone gray color.
In the case of pigment inks, it is considered that the color of the ink landed last is dominant because the inks are generally high in light blocking effect (apt to hide the background color). For example, it can be considered that when the ink landed last is a yellow ink, the tone is tinged with the yellow.
In the case of dye ink, the problem caused by the light blocking effect of the ink is indeed not significant, but a subsequent ink landing on a precedent ink may “spread”. Thus, the color of the ink landed first is rather dominant.
For this reason, there is a problem of a color difference formed like horizontal stripes (kind of so-called banding) within a sheet of recording subject due to a difference in recording direction during printing. In addition, there is another problem that the tone of a print obtained by bi-directional printing differs from the tone of a print obtained by unidirectional printing in spite of one and the same image data.
Generally in the ink jet recording apparatus, a plurality of kinds of recording paper can be used. The thickness may be not even among those kinds of recording paper. In addition, some recording apparatus can change the distance between the recording head and the recording paper. Further, the distance between the recording head and the recording paper fluctuates due to an error in assembling the recording apparatus.
For example, in a recording head for color printing, assume that an array of nozzle orifices for ejecting cyan ink (C), an array of nozzle orifices for ejecting magenta ink (M) and an array of nozzle orifices for ejecting yellow ink (Y) are provided in parallel in that order, and recording is carried out with the cyan ink (C), the magenta ink (M) and the yellow ink (Y) in that order during the motion of the recording head. In this case, recording is carried out with the yellow ink (Y), the magenta ink (M) and the cyan ink (C) in that order during the motion of the recording head.
Here, each color ink is ejected from each nozzle orifice onto the recording paper. When the distance between each nozzle orifice and the recording paper is not enough large, a so-called main droplet and a so-called satellite droplet are landed in a state in which the main and satellite droplets are not separated thoroughly but overlap each other.
The tone may change due to overlapping of the ink droplets of one and the same color, which droplets should be separated.
When ink droplets of the cyan ink (C) or the magenta ink (M) are superimposed on each other, the value of optical density linearly increases. In other words, in those colors of ink, a linear relation is established between the number of superimposed ink droplets and the increased value of optical density.
However, in the yellow ink (Y), a linear relation is not established between the number of superimposed ink droplets and the increased value of optical density, but the growth of the value of optical density rapidly saturated. As a result, the increase (growth) of the value of optical density of the yellow ink (Y) due to superimposed ink droplets is smaller than that of any other color ink. This phenomenon can be regarded as caused by the yellow color material ratio in the ink which ratio is higher than any other color material ratio because the coloring of the yellow color material is the weakest.
In such a manner, there is a difference in properties among the ink colors when ink droplets are superimposed on each other. This difference appears as a change of tone when the distance between each nozzle orifice and recording paper is not enough large.
FIG. 25 shows a specific example. In this case, when the distance (PG: Paper Gap) between each nozzle orifice and recording paper is not larger than 1.06 mm, a main droplet overlaps a satellite droplet so that a hue difference ΔE increases.